The present invention relates to a roll for use in a heat treating furnace, preferably, in a hearth roll with coatings for steel sheet carrying and installed in continuous annealing furnace for producing steel sheets and a method of producing the same. The roll has excellent build-up resistance, heat resistance and wear resistance. The roll operates well under respective atmosphere such as reducing atmosphere, nonoxidizing atmosphere, as a case may be weak oxidizing atmosphere.
When the metal sheet (hereinafter, referred to as "steel sheet") is heat-treated, a plurality of rolls (hearth roll) are installed in the heat treating furnace to carry the steel sheet.
The temperature in the heat treating furnace is controlled in accordance with the kind of steel sheet to be treated and the object, but the heat treating furnace recently operates at a temperature of not lower than 1100.degree. C.
The hearth rolls installed in such a heat treating furnace must support steel sheets under high temperatures, so that the hearth rolls are subjected to large frictional resistances. Therefore, such a hearth roll requires on its surface on excellent heat resistance and wear resistance.
Even in a high temperature circumstance, for example, in a hearth roll used in a heat treating furnace operating under reducing atmosphere, more excellent build-up resistance is required, since once the build-up is caused on the surface of the roll with excellent heat resistance and wear resistance, the carried steel sheet is in contact with this build-up, thereby causing press-scoring on the surface of the steel sheet, resulting in a decrease of value of product.
To overcome such a problem, Japanese Patent Laid-open No. 23,755/86 discloses a method of spraying ceramics of Cr.sub.2 O.sub.3 -Al.sub.2 O.sub.3 (Cr.sub.2 O.sub.3 : 70.about.90 wt %, Al.sub.2 O.sub.3 : balance) solid solution on the surface of the hearth roll. This technique improves pick-up phenomenon on the roll surface, but it has been found that when operating temperature becomes 900.degree. C. or more, a heavily ceramic sprayed coating is susceptible to peeling from the roll surface.
The Japanese Patent Laid-open No. 141,861/85 discloses a method of forming a sprayed coating on a hearth roll with the use of an alloy (Co: 35.about.55 wt %, Al: 3.about.20 wt %, balance: at least one of Cr, Ni, C, Ta, Y, Mo and Zr). This technique provides a sprayed coating with good adherence, but it is found that a build-up resistance under high temperature operation is not enough and there is effort to improve wear resistance.
The Japanese Patent Laid-open No. 81,236/74 discloses a high-temperature wear-resistant coated article, and a process for producing it, wherein the coated layer comprises metal oxide particles uniformly dispersed in a metal alloy matrix.
This article satisfies the following conditions.
(1) The above metal oxide particles are at least one selected from a group consisting of aluminum oxide, chromium oxide, beryllium oxide, calcium oxide, titanium oxide, niobium oxide, thorium oxide, zirconium oxide, tantalum oxide, silicon oxide, magnesium oxide, hafnium oxide, yttrium oxide, rare earth metal oxide, and a spinel combination of the above metal oxides.
(2) The above metal oxide particles are sized between about 0.05 micron and about 74 microns, and uniformly present in a volume fraction of between about 2% and about 50%.
(3) The metal alloy matrix comprises essentially at least one first metal selected from a group consisting of iron, cobalt and nickel, and at least one second metal selected from a group consisting of aluminum, silicon and chromium, the aggregate of the first metal is at least 40% by weight of the alloy and the aggregate of the second metal is between about 10% and about 40% by weight of the alloy.
(4) The thus obtained coated layer has surface hardness of at least 500VHN.
The sprayed coating obtained by the conventional technique improves heat resistance and wear resistance, but build-up resistance, in case of applying this sprayed coating on the hearth roll for use in the heat treating furnace under a reducing atmosphere and a non-oxidizing atmosphere, is not described at all. These conventional techniques disclose means for uniformly dispersing only metal oxide particles in a metal alloy matrix in order to improve heat resistance and wear resistance of the coated layer. However, it is difficult for such a coated layer to improve the desired build-up resistance. That is, carbide particles play an important role in an improvement for build-up resistance. The conventional techniques do not disclose the dispersion of carbide particles in the metal alloy at all but rather describe that carbide particle is an unsuitable particle for a coating reinforcing component.
As described above, the conventional roll provided with a coating for a heat treating furnace has excellent heat resistance, wear resistance and peeling resistance, but does not exhibit an excellent build-up resistance under high temperature reducible atmosphere. That is, there is a problem to be solved in that the conventional roll does not exhibit a well build-up resistance under a high temperature reducible atmosphere.